Understanding overlap in resting state fMRI networks at the single cell level: a cross-species approach Abstract Resting state functional connectivity MRI (rsfcMRI) is a popular tool to investigate the intrinsic functional organization of the brain into large scale networks. Multiple different lines of investigation have pointed to the importance of densely interconnected `hub' regions for cognition and behavior. However, the functional architecture of cellular circuits in these hub regions is unknown. To study the cellular underpinnings of hub regions, we bring together an interdisciplinary research team to bridge across species and across scales. We start by generalizing recent advances in human fcMRI analyses across species to characterize individualized patterns of network overlap in rsfcMRI data from awake macaque monkeys (Aim 1). This allows us to identify regions of interest for recordings in this same animals from a hub region where two (or more) networks spatially overlap, and from two non-hub regions that strongly contribute to only one of the networks respectively. We then ask whether, at a finer cellular scale, there is true neural coupling between both networks in hub regions, or whether networks that appear spatially overlapping at the resolution of rsfcMRI data are in fact spatially interdigitated rather than overlapping at a finer scale (Aim 2). Lastly, we use electrophysiological recordings to determine whether individual neurons in hub regions integrate information from both overlapping networks (i.e. coupling), or whether neurons dynamically switch their network allegiance from one network to another over time (Aim 3). The outcomes of this proposal have important implications for the modeling and interpretation of human rsfcMRI data. This R34 proposal provides the opportunity to establish a new collaboration and validate our methodology across species. These factors are essential for the next stage of our project, a Targeted Brain Circuits Project R01 proposal, in which we will build on this line of investigation by bridging into behavior to study how fundamental principles of the brain circuits in hub regions form the biological basis of mental processes.